1.0 Field of the Invention
A method and an apparatus is described for detection and tracking of one or more objects over land in the presence of land-based clutter, including low observable (LO) objects such as a humans, animals, vehicles, or small low-flying aircraft using an non-coherent radar or the amplitude output from a coherent radar.
2.0 Brief Description of Prior Art
Wide area surveillance and persistent surveillance of an area or a facility, or the borders of a country, like the Southern Border of the United States of America, typically requires 360-degree radar coverage to detect and track people, animals with people on or alongside, vehicles, and small, low-flying aircraft/ultra-lights approaching or crossing the border with an operationally acceptable probability of detection (PD) and probability of false alarm (PFA). Detection and tracking of low observable (LO) objects or targets with a radar is usually accomplished with a coherent radar using range-Doppler or moving target indication (MTI) processing methods. In U.S. Pat. No. 8,026,844, Fox, et. al., teaches how a non-coherent radar can be used for surveillance applications. The method of the present invention illustrates a method and an apparatus for surveillance for use in moderate to high clutter conditions with a non-coherent radar that has similar performance to a coherent Doppler radar, and in additional, has several additional advantages in terms of reduced cost, extended range, operation on a wide range of platforms with motion that makes coherent Doppler radars unusable, more robust tracking, and detection of stationary targets.
If detection and tracking of small, slow moving land targets, such as people and vehicles, is done with a non-coherent radar, a high level of performance can be achieved in terms of PD and PFA when the radar operates under system noise or low clutter conditions. Unfortunately, such conditions do not typically exist very much of the time. The performance of non-coherent radar degrades seriously if the radar operates in moderate to high clutter conditions. In fact, detection is often not possible and the PFA can become unacceptably high. Part of the reason for this poor performance is that target is masked by land clutter and by wind clutter. The method and apparatus of the present invention allows operation in these higher clutter conditions for non-coherent radar. The method used to enhance the performance for slow-moving targets in wind clutter also work to improve the performance of coherent Doppler radars.
The method and apparatus of the present invention apply to the low-cost, non-coherent radars operating at X-band and other frequencies of similar wavelength (e.g., K-, Ku, C-, S-, and L-band). Such radars have been used for marine navigation applications for many years, but they were not used for detection and tracking of targets over land such people, animals, vehicles, and small low-flying aircraft until the method and apparatus showed that this was possible (U.S. Pat. No. 8,026,844). The amplitude/intensity data obtained from a non-coherent radar have been processed to detect targets of interest in the presence of low clutter or no clutter (i.e., system noise). In strong clutter, typically found when there are a lot of scatters in the radar coverage cells and coverage area (such as uneven and irregular terrain, trees, bushes, buildings, rocks, boulders and outcroppings), the clutter is large enough to prevent the detection of the smaller targets like people. Coherent Doppler radars, which are significantly more expensive than the non-coherent radars described herein, could exploit the movement of these weaker targets and detect them provided they had sufficient radar sensitivity (i.e., low system noise), sufficient Doppler resolution to sense the movement velocity, and robust signal processing algorithms to operate against slow-moving targets.
The method and apparatus of the present invention allows the non-coherent radar to operate under strong clutter conditions. As described above, when the radar dwell times are long enough to produce and allow the target modulations to be measured and processed appropriately, non-coherent radars can be used to detect (or track) small targets like people or ultra-lights, as well as larger targets, such as vehicles or small low-flying aircraft. The preferred method of processing is to produce a Doppler spectrum and then identify targets by their Doppler (velocity) and/or their spectral strength. However, such target modulation can also be detected by bandpassing the data into velocity bins typical of the target motion and compare the intensity (e.g., the mean or median) or intensity fluctuations (e.g., variance or standard deviation) in the bin to that of the background and/or system noise.
The method of the present invention is based on the backscattering of radar pulses broadcast by non-coherent radars, where interference effects between radar pulses scattered by the objects of interest (targets) and the surrounding clutter environment lead to the production of exploitable exoclutter signal energy. This signal energy manifests itself as high frequency amplitude modulations that are distinctive and identifying of the target type, characteristic of the target motion as well as the motion of other targets in close proximity, and exceed the amplitude of the radar response of the background clutter environment within a distinguishing frequency band. The targets are mainly ground targets, but the same method also works for low-flying aircraft and ultra-lights.
We call this phenomenon “wavefront interference Doppler (WiDop).” Wavefront interference has been applied to optics and light for determining range and velocity, but, as stated below, this phenomenon has not been identified or exploited for radar applications for detecting and tracking ground targets, especially slow-moving or small targets such as people. This phenomenon has significant applications for detecting and tracking people, vehicles, and low-flying aircraft of all types with low-cost, non-coherent radars, because it allows the non-coherent radar data to be processed into a Doppler spectrum, like the Doppler spectrum obtained with a coherent Doppler radar. The Doppler spectrum obtained with the WiDop processing gives the strength and the velocity of the target relative to the clutter or system noise, but has an ambiguity in the target direction. This ambiguity is easily resolved from the target track; it could be also be resolved with a more complex time-frequency-range algorithm.
As illustrated in Table 1, there are four general types of radars and radar processors to detect moving targets. They are (1) Moving Target Indication (MTI) processors for coherent Doppler radars (coherent MTI), (2) Doppler spectral processing for coherent Doppler radars (coherent Doppler), (3) MTI processors for noncoherent radars (non-coherent MTI), and (4) Doppler processing for noncoherent radars (non-coherent radar). Coherent radars use a phase reference produced by a stable oscillator to acquire phase and amplitude data. The coherent MTI processor identifies all moving targets by filtering out all non-Doppler shifted signals. A coherent Doppler processor produces a Doppler spectrum, which allows not only the target to be detected by the strength of the spectrum at each Doppler bin but is also capable of determining the magnitude and sign of the radial speed of the target in a similar way as done by coherent MTI and coherent Doppler radars. Unlike a coherent Doppler radar, the phase of the transmitted signal is unknown for a non-coherent radar, but in the presence of land clutter, the beating between the target and the land clutter provides a Doppler shifted target signal relative to the clutter. Thus, non-coherent MTI and non-coherent Doppler processors can be used. However, only the amplitude information is obtained; no phase information is obtained. Non-coherent Doppler is nearly identical to coherent Doppler processor except the sign of the radial speed is unknown. Non-coherent MTI processors have been used on airborne radars because the relative motion between the airborne radar and the target signal and the clutter was the same and removed as part of the MTI processing. Coherent radar, both coherent MTI and coherent Doppler systems have been and continue to be widely used. In contrast, non-coherent Doppler processor system have not been widely used, and more importantly, have not been used or demonstrated for land targets other than as described in U.S. Pat. No. 8,330,647.
TABLE 1Four general types of radar and radar processorsdetecting and tracking targetsProcessorCoherent RadarNon-coherent RadarDopplerCoherent Doppler (1)Non-coherent Doppler (2)Moving TargetCoherent MTI (3)Non-coherent MTI (4)Indication (MTI)
In general, the best radar in terms of performance, everything else being equal, is a coherent radar, and the best processor considered to be a coherent Doppler processor. Thus, the best performance is (1) and the worse performance is (4). The method and apparatus of the present invention called WiDop (wave-front interference) is similar to the coherent Doppler radar and processor system, but has a number of advantages over a coherent radar in terms of cost, range, operation with platform motion, tracking, and detection of stationary targets.
Non-Coherent MTI.
Non-coherent MTI is coherent MTI without phase. It was invented very early in the history of radars. During WWII “the MIT Radiation Laboratory, set up “Project Firefly” to develop a non-coherent Airborne MTI radar (AMTI) for battlefield use. The resulting radar, the AN/APS-27,” had a number of deficiencies [5]. Additional examples of non-coherent MTI implementations are the AN/TPN-19 and AN/UPS-1 radars [6-7]. Skolnick [2] explains non-coherent MTI as “externally coherent.” Schleher [3] referred to “clutter referenced MTI.” Skolnick [4] describes it as the “target vector beating with the clutter vector.” All of the early applications were for separating aircraft from weather and ground clutter. Skolnik specifically states “the present discussion will be confined to airborne radar” since the concept does not hold well for ground targets and ground clutter discretes. In fact discrete clutter are recognized as especially difficult for non-coherent MTI.
Kretschmer et. al. [8] provide a detailed analysis of coherent vs. non-coherent MTI. Their FIGS. 1 and 3 lead to the following observations. (Note: panels a-b in FIG. 1 are switched.) The MTI performance depends on the length of the MTI canceller, the spectral width of the clutter, and whether or not the radar is coherent or non-coherent. Both coherent and non-coherent can provide very impressive gains against clutter, but coherent will generally do better. For example, with a 5-point MTI canceller the non-coherent MTI may provide a respectable 10 to 20 dB gain. Under the same conditions, coherent MTI provides an additional 5 to 10 dB gain or more. Non-coherent MTI processors are no longer used, mainly because of their poor performance relative to coherent MTI.
Non-Coherent Doppler.
Holm and Echard [9] provided a theoretical analysis of the two basic radar and two basic processor types in Table 1, including a non-coherent Doppler processor similar to WiDop. Theoretical SNR gains were provided with the conclusion that non-coherent Doppler is 10 dB better than non-coherent MTI and 3 dB less than coherent Doppler. The 3 dB difference is valid only when clutter and target are narrowband and are well separated such that the WiDop spectrum of the signal is entirely riding on receiver noise. The more general case with the target overlaps the clutter spectrum requires a more detailed analysis than performed by Holms-Echard.
Table 2 illustrates the relative difference in performance for all four radar/processor combinations based on the HolmEchard's theoretical estimate of SNR. It was assumed that non-coherent Doppler (WiDop) is arbitrarily set at an SNR of 30 dB as a reference point. It is further assumed that the SNRs are for a specific set of conditions and parameters and for simple, well separated target and clutter spectra. For real world conditions the SNR will be different and the Doppler vs MTI differences will be even more dramatic.
TABLE 2SNR for 4 general types of radars and radar processorsdetecting and tracking targetsProcessorCoherent RadarNon-coherent RadarDopplerCoherent Doppler (1)Non-coherent Doppler (2)33 db30 dBMoving TargetCoherent MTI (3)Non-coherent MTI (4)Indication (MTI)25 dB20 dB
The paper by Holm-Echard is a theoretical mathematical analysis of two general types of radars and two general types of radar processing that were performed with idealized conditions and simplified assumptions. The paper did not indicate that any of the methods were actually demonstrated or implemented operationally, nor would they work for any of a large number of different radar platforms (airborne, space, ship, vehicle, or land platforms) and/or a large number of different types of targets (space, airborne, land, or ocean targets). In fact, at the time of the paper (1982), non-coherent Doppler processing was not being used and had not been applied to land targets. This is still true today except for the method and apparatus of the present invention. In fact, coherent Doppler processors were the main radar system being used for target detection problems, including and especially land targets.
The present invention provides a method and apparatus to detect and track low observable land objects such as a humans, vehicles, and small low-flying aircraft in the presence of land clutter, particularly those objects moving at low velocities relative to the radar look angle, using an non-coherent radar or the amplitude output of a coherent radar. WiDop exploits the beating phenomenon that occurs between a moving object and the land clutter. As stated above, this phenomenology was identified for non-coherent radars over 50 years ago for airborne radars, but it has never been applied to low observable land targets. In fact, the approach was confined for use on only airborne radars, because Skolnik concluded that the concept does not hold well for ground targets and ground clutter discretes. The approach was called non-coherent radar MTI, which found application on airborne radar systems for detecting and tracking aircraft, because it has the advantage that the airplane speed is automatically removed because the clutter and the target have the same phase reference. The method and apparatus of the present invention does not use MTI methods. It utilizes range-Doppler processing methods.
There are many reasons why this phenomenology has not been exploited using non-coherent radars for land targets, particularly small and slow moving targets like people. With the advent of coherent Doppler radars near the end of World War II and the high performance achieved with these radars for land targets, which is much higher than for non-coherent radars, there has been no motivation to develop a WiDop system for land targets.
An important motivation for the present invention is cost, because of the need for a large number of such radars for persistent and border surveillance applications for defense, intelligence, and homeland security applications. However, there are other important advantages besides cost that make the use of non-coherent radars very attractive. First, using WiDop, a non-coherent radar can be made to perform nearly as well as a coherent radar, but a non-coherent radar can operate at longer ranges because of its higher operating power, track better because of its high sample rate, detect stationary targets, which is not possible with a coherent Doppler radar, and operate on platforms without the adverse affects of platform motion. This latter advantage is particularly important, because the non-coherent radar using WiDop is immune to mechanical motion and phase instabilities that have made many of the coherent Doppler radars completely unusable on tower and aerostats because they have not implemented motion compensation. It is also important to note, that without the use of WiDop described herein, non-coherent radars will not perform well. This patent demonstrates with field data that the phenomenology will work for the proposed land objects and suggests some methods to significantly enhance WiDop. The method of WiDop is critical to the performance of non-coherent radars for detection and tracking of targets in clutter.